AVS2 carb tuning. Anyone with experience out there?
One problem I see with the air door is when it opens it slams open. It would be nice if it were more gradual.
It's made that way on purpose, but if it really-really slams open, then the tension is quite a bit too loose, hence my recommendation for 2 additional turns.
If you can feel the secondaries opening, then the tension is too loose.What you are feeling is the drop in air speed/flow and the subsequent sag in power, while all the circuits re-adjust to the new airspeed, and the secondary fuel flow comes on line. The secondaries don't actually need to be fully open until peak torque, which is gonna be around 4000rpm with that cam.
The air-door should be tight enough to not be able to open when you slam the throttle open in Neutral/Park at idle. But same action at 3000 should get her started. Try 2 turns tighter; whatever is necessary to get the bog gone; you can always loosen it later, as your tune gets sharper..
Do not fall for the idea that the secondaries need to open ASAP; that's just not true with that carb. The power delivery needs to be seamless. If you loose a half second on the transition, you end up slower in the speed contest.
You gotta remember that the engine does not suck air in.
The atmosphere pushes air in, trying to fill the low-pressure area in the cylinders that the falling pistons and ring-seal is creating.( see note 1)
As the rpm rises, and with only the primaries open, the air flow thru the throttle bores picks up ever increasing speed. Let's say at 3000rpm, the engine is processing 300cfm, thru the primaries. If you then slam the secondaries open at 3000rpm, the engine is still only gonna process 300 cfm, which is all it can do at 3000 rpm. Now all 4 throttle-bores are sharing the same 300 cfm.Subsequently the airspeed thru the primary throttle bores HAS to drop.
When that happens, fueling in the primaries, drops in proportion, back to 150 cfm level. While in the secondaries, fueling is just starting up.
So your job is to make the transition between primary flow only, to shared flow, as smooth as possible;
The easiest way to do that, is to prevent the secondaries from slamming open.
The second, is to replace the Vsec, with a double-pumper.
The third, is to get a higher-stall TC, so that the sag happens with no load on the engine; try a 3500,lol. or for street; maybe a 3000.
Or you can do a combination; like a 2800TC and 3.73gears, and a 650DP.
Or you can just slow the daymn secondaries down........... lol
Note1
Talking about ring-seal. Some guys , are under the impression that leakdown is not "bad" until around 10%. Well; that may be acceptable for some ..... on the compression side of what the rings do.
But the rings also have to create the low-pressure in the cylinders, to cause atmosphere to move in that direction. So the poorer the ring-seal, the less air will find it's way into the cylinder, and the faster the engine spins the bigger the problem will be, and the more intake duration will need to overcome it.
Don't be misled, 10% ischit. 4% is a good and reasonable target. Less is better.
Course if you have a 440, I guess you could afford 10%,lol.
Another thing to keep in mind is if you have a 440, the total volume in that chamber, swept plus chamber,etc; could be as high as say 1030cc, with the piston parked at the bottom.
Whereas the 273 might have say 620 total. But after you stick a dome into the 273,and after the late Ica on the big-cammed 273 finally closes, maybe 420cc .................. Now; if the ringseal is down 10% ..............................
This is what makes the modest-cammed 360 so attractive to me; 830cc with flat-tops, and maybe 650/700 after the intake closes. That is able to make a lotta torque right where a streeter needs it.
In comparison;
the 340 could be 770cc/660 after the intake closes.and
the 318 could be 730/580.
Engines are airpumps.
To make power it takes rpm to process sufficient air.
But to make street-torque, you gotta be able to process air at lower rpms. 660 is a suitable target, allowing you to run less gear, say 3.23s.
Whereas 580, being 12% less than 660, might require 12% more gear, mathematically 3.68s, which round to 3.73s.
